3.3 Microscopic examination
Many interesting features can be found on failed samples by inspection with a hand magnifier. The features can be photographed with close-up attachments for a single-lens-reflex camera, such as extension tubes and bellows. However, detailed inspection of many features demands the use of a simple optical microscope, such as a binocular microscope with a magnification up to about × 60. The microscope can show scratches, abrasion, hairline cracks, crazes and contamination debris that might have been missed and discounted. If the sample is fractured, a detailed map from the microscope of the fracture surface is helpful in determining the sequence of events during crack propagation.
When surveyed, the fracture surface can show many specific features that indicate how crack growth occurred. Features such as the nature and direction of crack growth, and the age of the fracture can all be studied using a technique known as fractography.
Figure 23 shows the fracture surface of a polyacetal pipe fitting, which caused a flood when it finally parted.
The surface had clearly been formed a considerable time before water escaped, judging by the deposits that were present on most of the fracture surface. A fresh fracture showed where the sample finally parted when it rolled off the desk of the insurance investigator's desk after it had been removed from the accident site!
There were at least two zones of different colouration of deposit, which turned out simply to be inorganic salts from the local hard water supply (Figure 24a).
However, the deposit could not be removed to examine the underlying fracture in the polymer surface itself. This is a frequent problem with unique failed samples: several investigators may need to examine the surface independently, so any inspection must be carried out non-destructively.
However, subsidiary features can be used to work out where the origin, or origins, of the failure occurred on the surface. Where the pipe fitting was in service, several cracks must have started independently, because remnants of crack growth were detected at the outer edge of the fracture. The remnants consisted of flaps of material left behind after growth and caused by crack branching into the interior of the material. They indicated crack growth direction, so by surveying the whole edge, it was possible to infer there must have been several separate cracks. The flaps pointed in different directions, so must have been formed by separate and independent growth, enabling a more detailed map of the features to be constructed (Figure 24b). The cracks had grown from the base of a screw thread, a geometric feature that is a well-known stress concentration. Small weld lines were present here, so the combination allowed small applied stresses to have been magnified many times until the strength of the polymer had been exceeded. This is a common situation found in failed samples, where different defects combine together to produce failure. Small weld lines on the inner bore also probably grew to merge with the outer cracks. As you will see later, however, an additional factor was also present that produced an early fracture, but a late failure, in this particular sample.
Sometimes it is useful to carry the investigation further using scanning electron microscopy. This method uses an electron beam instead of a beam of visible light, and magnifies features that have molecular dimensions.
In addition, direct chemical analysis of any impurities, for example, is possible from the X-ray spectrum emitted by the sample. Box 10 relates how the method was used to solve a problem encountered on the Hong Kong underground railway.
Box 10 Problem on the Hong Kong underground
The makers of the large lead-acid storage batteries used to power the underground locomotives wanted an independent investigation of a fire on the then new Hong Kong underground railway. The base of one of the storage batteries showed a large hole that was surrounded by burnt polypropylene (Figure 25). A close-up showed that a brittle crack bisected the hole (Figure 26).
Acid had leaked from the crack and caused considerable damage to the metal container. The shape of the hole seemed regular as though caused by impact with a hard object, as the SEM image of Figure 27 shows. SEM analysis of the edge of the hole showed the presence of aluminium and silicon elements, as well as the lead and sulphur expected from the inner plates of the battery (Figure 28).
The X-ray spectrum allows particular elements to be analysed from the peak position. The height of each peak reflects the concentration of that element present. There were substantial traces of aluminium and silicon present compared with slight traces of lead and sulphur. The traces were probably caused by an aluminium alloy stud present in the base of the container when the battery was inserted (Figure 29). The stud should not have been there at all. When the base of the battery impacted the stud, the impact damage initiated a crack, which then penetrated the interior of the cell, allowing acid to leak out slowly. It reacted with the aluminium and removed the evidence of the impact, apart from the traces left on the edges of the hole. The subsequent fire damage was not the fault of the battery manufacturer but the installer, who should have checked the floor of the holder was clear of debris before inserting the battery.